Development of a fibre-optic sensor for position monitoring of vertex detectors in High Energy Physics experiments

The demanding task of monitoring the spatial position of vertex detectors used in high energy physics experiments performed in particle accelerators requires sensors able to measure linear displacements with high resolution under strong magnetic and radio frequency fields. Due to the severe environmental conditions, traditional electric transducers are hindered from working satisfactorily. This paper describes the development of a packaged opto-mechanical sensor which can overcome these problems. The principle of operation relies on the measurement of the tangential strains of a CFRP -shaped elastic element by means of an optical in-fibre Bragg grating (FBG): its design allows us to measure linear displacements with high accuracy and extended linear range. The sensor is self-compensated in temperature in order to take into account wavelength shifts due to apparent strains and to increase its intrinsic accuracy. The latter has been evaluated by calibration performed in the laboratory to be within ±3 pm, corresponding to ±13 μm, which is close to the resolution of the interrogation system used to detect the FBG reflected wavelength shift.